A unilateral ceasefire announced by the Colombian FARC guerilla is due to come into force on Saturday, but the group was linked to a deadly attack and an international drug bust on Friday.

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The Marxist-led FARC guerrilla killed five soldiers in an ambush just hours before the rebels were to begin the indefinite ceasefire, the army said.

The Revolutionary Armed Forces of Colombia (FARC) ambush, in which several soldiers were also wounded, occurred in the southwestern province of Cauca as the army patrolled a highway damaged in a bomb attack a short while earlier.

The soldiers were killed by explosions and machine-gun fire, the army said.

The FARC unexpectedly announced on Wednesday that it would cease hostilities for an unlimited time from Saturday and engage in hostilities only if its fighters came under attack.

The government, however, has rejected a rebel demand that an independent international organisation monitor the ceasefire and cautioned that a bilateral truce could allow the rebels to re-arm.

The FARC has not commented on the plan since, casting doubt over whether it will keep its promise of suspending hostilities.

The commander of the brigade whose unit was attacked, General Wilson Chawez, vowed the army would continue fighting the "terrorists".

"We're going to pursue our military operations. It's a unilateral ceasefire. We're an army that remains on the offensive," he told journalists.

FARC linked to Panama drug ring

In a separate development, neighbouring Panama said on Friday it had arrested 59 people as part of an investigation into a drug trafficking ring with links to the FARC and Mexico's Sinaloa cartel.

Three police officers were also killed on Friday in an attack by Colombia's smaller ELN guerrilla group.The policemen died in a separate attack in northeast Norte de Santander province, which police blamed on the National Liberation Army, or ELN.

The 18-month investigation into the organisation, which used Panama as the base for its smuggling operations, led to the seizure of drugs, cars, speed-boats and cash, and the discovery of various bank accounts used to launder drug money, Javier Caraballo, Panama's anti-drug investigator, said in a press conference.

The FARC was founded in 1964 to fight for rural land reform but has been deeply involved in the narcotics trade.

The government of Colombian President Juan Manuel Santos has engaged in slow-moving peace talks with the FARC since late 2012 in an effort to end a five-decade-old conflict that has claimed some 220,000 lives and left millions uprooted from their homes.

As part of the negotiations, the FARC has already agreed to put an end to the illegal drugs trade.